fix: add forward slash between slug and url for admin portal url

[brobro10000]

Description:
This pull request makes a small but important fix to the formatting of the admin portal URL for fulfilled checkout intents, ensuring a slash is present between the base URL and the enterprise slug. It also updates the related test to match the new URL formatting.

See the prod and stage config files for validation.

• URL formatting fix:

  • Updated the admin_portal_url property in customer_billing/models.py to include a slash (/) between ENTERPRISE_ADMIN_PORTAL_URL and enterprise_slug, ensuring correct URL construction.

• Test update:

  • Modified the test in test_models.py to use the base URL without a trailing slash, aligning the test with the new URL formatting logic.

Jira:
ENT-XXXX

Merge checklist:

• ./manage.py makemigrations has been run
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.

Post merge:

• Ensure that your changes went out to the stage instance
• Deploy to prod instance